1. Field of the Invention
The present invention relates to a magnesium alloy member, and in particular to a magnesium alloy member including an anodic oxidation coating. The present invention also relates to a method for producing such a magnesium alloy member and a transporter including such a magnesium alloy member.
2. Description of the Related Art
Conventionally, steel has widely been used as a material for transporters because of superior mechanical properties, superior processability and low cost thereof. In order to improve the fuel efficiency and running performance, however, transporters are desired to be more lightweight. Research has been made to use materials more lightweight than steel.
Recently, low-cost refining methods for titanium, aluminum, magnesium and the like, which have a lower specific gravity than that of steel, and methods for producing alloys containing such metal materials have been developed. Technologies for improving the strength and processability of alloys of such metal materials have also been developed.
In such a situation, it has been proposed to use alloys of titanium, aluminum and magnesium as materials for members of transporters. Particularly, when magnesium alloys are used, the weight of the transporters can be significantly reduced because the density of magnesium is about 23% of that of steel.
However, magnesium alloys are more likely to be corroded than aluminum alloys in certain environments. As one technique to improve the corrosion resistance of magnesium alloys, an anodic oxidation coating is formed on a surface of a magnesium alloy.
An anodic oxidation coating on an aluminum alloy is known to include a porous layer and a non-porous barrier layer. These layers can be observed by an electron microscope. An anodic oxidation coating on a magnesium alloy also includes a porous layer and a barrier layer as disclosed in Japanese Laid-Open Patent Publication No. 2006-291278.
This publication describes that the corrosion resistance of magnesium alloys can be improved by reducing an average diameter of micropores in a surface area of the porous layer from that in the conventional art to 100 nm to 25 μm.
However, transporters are mainly used outdoors and therefore members forming the transporters are often exposed to severe environments. Hence, magnesium alloys are desired to have more improved corrosion resistance.
Most of the magnesium alloy members practically used today are used for domestic electronic appliances, particularly for reducing the weight of small mobile devices. The magnesium alloy members for these applications are small interior components and are not required to have such a high corrosion resistance as is required of those used for transporters.
In general, as the anodic oxidation coating is thicker, the corrosion resistance is higher. An anodic oxidation coating formed on a magnesium alloy member used for domestic electronic appliances often has a thickness of about 5 μm to 15 μm. When an anodic oxidation coating of such a thickness is formed on a magnesium alloy member for transporters by a conventional technique, a sufficient corrosion resistance is not provided. Studies performed by the present inventors have found that a thickness exceeding 15 μm is required in order to guarantee a sufficient corrosion resistance for a magnesium alloy member used for transporters.
However, when the anodic oxidation coating is thickened, the porous layer is also thickened accordingly. A porous layer, which is mainly formed of magnesium oxide (MgO) or magnesium hydroxide (MgOH), has a convex and concave surface and thus is more brittle than the magnesium alloy which is the starting material. When the porous layer is thickened, and the height difference between the convex area and the concave area becomes large, such a location with a large height difference is likely to cause fatigue destruction and thus decrease the fatigue strength.